1. Field of the Invention
The present invention relates generally to wireless communications systems.
2. Description of the Related Art
The performance of modern wireless communications equipment is typically limited by three factors: power & bandwidth, interference, and the wireless channel. The Federal Communications Commission has imposed power and bandwidth limitations on most communications bands, including the 2.4 GHz industrial, science and medicine (ISM) and the 5 GHz Unlicensed National Information Infrastructure (U-NII) bands. Fundamentally, these power and bandwidth limitations limit the range and data rate of any device deployed in a wireless network. Because of these limitations, devices that only employ conventional modem and signal processing technologies may not be able to support future multimedia and other high data rate applications such as streaming video.
In the unlicensed bands, interference is a serious problem since multiple emitters may share the same frequency in an uncontrolled manner. The interference caused by other emitters ultimately reduces the throughput possible from a system. Finally, the wireless channel limits overall performance due to large-scale propagation loss, multipath delay spread and temporal channel variation.
Published communication techniques include error correction coding, advanced modulation techniques, channel equalization, diversity, and beamforming. More recent approaches appear to be concentrated in the area of space-time coding. Error correction coding and modulation are the most common techniques used to mitigate the effects of path loss, interference and multipath fading. At some point, though, their use often introduces unacceptable overhead (due to added redundancy), latency, cost, and complexity. For example, the receiver phase noise required to implement ≧256-QAM (quadrature amplitude modulation) may require components whose costs are not acceptable for low-cost consumer devices. New turbo codes that exhibit enhanced performance require multiple decoding iterations, thus requiring higher speed, more costly circuitry to implement.
Channel equalization and spatial techniques such as switched diversity improve the performance of a device operating in a multipath fading environment, but do little to combat interference. Conventional beamforming algorithms based on direction-of-arrival (DOA) estimation are not feasible for systems with significant multipath, such as in most indoor applications. Space-time coding requires only a slight increase in receiver complexity for two antennas, but does not improve the performance of a system against interference. For more than two antennas, the implementation cost of space-time coding may prevent its adoption in systems that require low-cost consumer devices. Additionally, space-time coding requires changes in both the transmitter and the receiver.